The invention relates to a support device for a drum which is provided with two journals whose centrelines coincide substantially with the centreline of the drum, which journals project from the drum on either side of the drum and each have their bearing in a bush attached to a frame so that they are rotatable about a common axis of rotation which substantially coincides with the said centrelines.
The wish in many practical situations is for a drum to be able to rotate around its centreline within very narrow tolerance limits. This is the case, for example, when the drum serves to clamp a flexible plate-shaped workpiece which, after being clamped, is machined by means of a feed motion of a tool holder carrying a tool fastened to it in a radial direction relative to the centreline of the drum and by means of a rotational movement of the drum. If the workpiece is clamped around a drum with a high resistance to bending, a rigid cylindrical body is obtained which renders an accurate machining of the workpiece possible. In particular, so-called masters for the manufacture of projection screens in the form of so-called linear Fresnel lenses for projection television are manufactured in this way. In this process, any deviations in the circular movement made by a point on the outer circumference of the drum must remain below one micrometer.
If such a requirement is to be met, it is impossible in general to use ball bearings, as is the case in conventional supports, but it is necessary to use radial fluid bearings which can operate between an outer circumference of a journal and an inner circumference of a bearing bush attached to the frame with small gaps of the order of approximately 12 .mu.m. In addition, the use of radial fluid bearings, in contrast to the conventional ball bearings, makes it possible to accommodate any expansion differences in axial direction between the drum and the frame in a simple way in that the journals can be axially shifted in the bearing bushes.
Now a first problem which arises here, especially in the case of a support device having a relatively long drum, is that the bushes surrounding the journals must be aligned relative to one another with great accuracy so that the centrelines of the said bushes coincide to within a very narrow tolerance and the two journals can be positioned in the bushes with a degree of clearance sufficient for a correct journalling action. In addition, a second problem with such long drums is that the centrelines of the two journals do not coincide. Generally, the axis of rotation about which the drum is rotatable does not coincide with the centreline of the drum in such a case, so that the rotational movement of the drum is inaccurate and the said axis of rotation also fails to coincide with the centrelines of the journals, so that also the rotational movement of the journals relative to the bearing bushes is incorrect, which may lead to an incorrect journalling action.